JPWO2019244830A1 - Carbon fiber and method for producing the same - Google Patents

Abstract

An object of the present invention is to obtain a carbon fiber that does not easily break during the molding process of a carbon fiber reinforced composite material and exhibits an excellent elastic modulus of the carbon fiber reinforced composite material. A carbon fiber having a strand elastic modulus of 360 GPa or more, a strand strength of 3.5 GPa or more, a single fiber diameter of 6.0 μm or more, and one or more of the following requirements. (A) When one end is a fixed end and the other end is a free end that can rotate about the axis of the fiber bundle, the number of remaining twists is 2 turns/m or more. (b) A single carbon fiber The total fineness, which is the product of the fiber fineness (g/km) and the number of filaments (pieces), is 740 g/km or more. Further, it is a carbon fiber in which the single fiber elastic modulus Es (GPa) and the loop breaking load A (N) satisfy the relationship of the formula (1). A≧−0.0017×Es+1.02 Formula (1) Further, the single fiber diameter is 6.0 μm or more, and the strand elastic modulus E (GPa) and the heating loss rate at 450° C. are 0.15% or less. The carbon fiber has a relationship with the knot strength B (MPa) evaluated in Equation (2) and has a twist number of 20 to 80 turns/m. B≧6.7×109×E-2.85 Formula (2) [Selection diagram] None

Description

The present invention relates to carbon fiber and a method for producing the same.

Carbon fiber has excellent specific strength and specific elastic modulus, and by using it as a reinforcing fiber of a carbon fiber reinforced composite material, it is possible to significantly reduce the weight of the member, making it an essential material for realizing a society with high energy efficiency. It is used in a wide range of fields as one. In recent years, application has been advanced in fields where there is a strong demand for cost reduction, such as automobiles and electronic device housings, and there is a strong demand for reduction of the final member cost including the molding cost.

In order to effectively reduce the cost of the final member, it is important not only to reduce the cost of the carbon fiber itself, but also to reduce the required amount by improving the performance of the carbon fiber and the molding cost by improving the molding processability. is there.

However, when aiming to reduce the amount of carbon fiber used while maintaining rigidity, which is one of the important characteristics as the final member, for example, simply applying the existing high elastic modulus carbon fiber does not necessarily mean the cost of the final member. Often does not go down. This is because the existing high-modulus carbon fiber has low productivity and tends to be expensive, and the low processability of molding tends to increase the total processing cost up to the final member. Moldability of carbon fiber is, for example, good handleability as a yarn bundle, difficulty in fluffing, and ease of thread connection required when switching carbon fiber bobbins when continuously producing carbon fiber reinforced composite materials. It is determined by the handling and processability in various processes until it becomes the final member.

Further, in recent years, the number of cases in which carbon fibers are used as discontinuous fibers is increasing, particularly for applications that place importance on cost reduction. Generally, when carbon fibers are used as discontinuous fibers, the fiber length of the carbon fibers tends to be shortened due to shearing or bending in the molding process. The existing high-modulus carbon fiber has such a strong tendency, and even if the tensile modulus of the carbon fiber is high, the rigidity of the final member is not effectively improved accordingly.

The most widely used polyacrylonitrile-based carbon fiber is a flame-resistant process of converting a carbon fiber precursor fiber into a flame-resistant fiber in an oxidizing atmosphere of 200 to 300° C., and a carbon in an inert atmosphere of 300 to 2000° C. It is industrially manufactured through a carbonization process that transforms into carbon. Further, the polyacrylonitrile-based high elastic modulus carbon fiber is industrially manufactured through a graphitization step of graphitizing in an inert atmosphere at a maximum temperature of 3000°C. Such a graphitization step can effectively increase the tensile elastic modulus of the carbon fiber, but on the other hand, equipment corresponding to high temperature is required or the crystal growth in the carbon fiber is promoted, so that the resulting carbon fiber can be obtained. It tends to have low tensile strength and compressive strength. And, such a high elastic modulus carbon fiber tends to have low productivity and productivity as a carbon fiber described above, and moldability in obtaining a carbon fiber reinforced composite material, and a fiber length when used as a discontinuous fiber. It tends to be short.

Several methods of increasing the tensile elastic modulus of carbon fibers by methods other than graphitization have also been proposed. As one of them, a method of applying high tension in the production process of carbon fibers has been proposed.

Patent Documents 1 and 2 propose a technique in which generation of fluff can be suppressed by controlling the molecular weight of the polyacrylonitrile copolymer even when high tension is applied in the carbonization step.

Patent Document 3 proposes a technique of increasing the strand elastic modulus by performing high stretching in the flame resistance step and the preliminary carbonization step.

Further, Patent Documents 4 to 7 propose techniques for improving process passability in a carbonization process by adding entanglement to a carbon fiber precursor fiber bundle and Patent Documents 8 and 9 for adding twist.

In Patent Document 10, by controlling the trial length dependency of the pre-carbonized fiber bundle by entanglement or twisting and carbonizing it with high tension, the strand elastic modulus of the obtained carbon fiber is increased, and the carbon fiber and the matrix are increased. A technique for suppressing a decrease in adhesiveness with is proposed.

Patent Document 11 proposes a carbon fiber having high knot strength and excellent moldability by controlling the copolymerization composition of the carbon fiber precursor fiber bundle even if the single fiber fineness is large.

Similarly, Patent Document 12 proposes a carbon fiber in which deterioration of mechanical properties is suppressed even if the diameter of a single fiber is large.

International Publication No. WO2008/047745 JP, 2009-256833, A International Publication No. WO2008/063886 JP 2001-49536 A JP, 10-195718, A JP, 2000-160436, A Japanese Patent Publication No. 47-026964 JP-A-56-091015 JP, 2002-001725, A JP, 2014-141761, A International Publication No. WO2013/157613 International Publication No. WO2013/157612

However, the conventional techniques have the following problems.

In Patent Documents 1 and 2, the molecular weight of the polyacrylonitrile copolymer is controlled, but the effect of improving the critical stretching tension in the carbonization step due to this is small, and a large improvement in the strand elastic modulus cannot be expected.

In Patent Document 3, although the stretch ratio up to the preliminary carbonization step is set high, the stretch ratio in the carbonization step, which easily improves the strand elastic modulus of the carbon fiber, is low, and a large improvement in strand elastic modulus cannot be expected. It was

In Patent Documents 4 to 9, no attention is paid to increasing the stretching ratio in the carbonization step, and there is no idea to pay attention to them.

Patent Document 10 shows that the strand elastic modulus, the adhesiveness to the matrix, and the strand strength can be compatible at a high level, and that the passability in the carbonization step was also good. However, no attention has been paid to the molding processability when obtaining the carbon fiber reinforced composite material and the fiber breakage when used as the discontinuous fiber, and there has been no idea to pay attention to them.

In Patent Documents 11 and 12, no particular attention is paid to the draw ratio in the carbonization step, and in the examples, the strand elastic modulus is increased up to 343 GPa by increasing the carbonization temperature. Although not mentioned, conventional approaches to increasing carbonization temperatures tend to result in poor processability in obtaining carbon fiber reinforced composites, similar to commercially available high modulus grade carbon fibers. Further, no attention has been paid to fiber breakage when it is used as a discontinuous fiber, and there is no idea to pay attention to them.

In summary, the prior art describes a method of achieving a high level of both the tensile elastic modulus of carbon fiber and the formability, and the ease of maintaining the fiber length when used as a discontinuous fiber. However, in order to realize a total cost reduction as the final member, it was an issue to obtain a method for achieving both at a high level.

In order to achieve the above object, the first aspect of the carbon fiber of the present invention is a carbon fiber having a strand elastic modulus of 360 GPa or more, a strand strength of 3.5 GPa or more and a single fiber diameter of 6.0 μm or more. In addition, the carbon fiber satisfies the following requirement (a) or (b).
(A) When one end is a fixed end and the other end is a free end that is rotatable with respect to the axis of the fiber bundle, the number of remaining twists is 2 turns/m or more. (b) A single carbon fiber The total fineness, which is the product of the fiber fineness (g/km) and the number of filaments (pieces), is 740 g/km or more.

The second aspect of the carbon fiber of the present invention is the carbon fiber in which the single fiber elastic modulus Es (GPa) and the loop breaking load A (N) satisfy the relationship of the formula (1).
A≧−0.0017×Es+1.02... Formula (1)
The third aspect of the carbon fiber of the present invention is that the single fiber diameter is 6.0 μm or more, and the knot strength evaluated when the strand elastic modulus E (GPa) and the heating weight loss rate at 450° C. are 0.15% or less. The carbon fiber has a relationship with B (MPa) that satisfies the expression (2) and has a twist number of 5 to 80 turns/m.
B≧6.7×10 ⁹ ×E ^−2.85 ...Equation (2)
Further, the method for producing a carbon fiber according to the present invention, the carbon fiber precursor fiber bundle is subjected to a flameproofing treatment in a temperature range of 200 to 300° C. in an air atmosphere, and the obtained flameproofed fiber bundle is subjected to an inert atmosphere. Among them, preliminary carbonization is performed by performing heat treatment at a maximum temperature of 500 to 1000° C. until the density becomes 1.5 to 1.8 g/cm ^3, and the obtained precarbonized fiber bundle is heat treated in an inert atmosphere. A method for producing carbon fiber for carbonization, wherein the single fiber fineness of the carbon fiber precursor fiber bundle is 0.9 dtex or more, the tension during carbonization treatment is controlled to 5 mN/dtex or more, and ) Or (d) is satisfied.
(C) The number of twists of the fiber bundle to be subjected to carbonization treatment is 2 turns/m or more. (d) The total fineness, which is the product of the single fiber fineness (g/km) and the number of filaments (pieces) of the resulting carbon fiber. 740 g/km or more

The carbon fiber of the present invention is a carbon fiber that has both excellent tensile elastic modulus and processability for forming a composite material, and can easily maintain the fiber length even when used as a discontinuous fiber. The carbon fiber of the present invention is effective in reducing the required amount of carbon fiber, improving the productivity of composite materials, and improving the mechanical properties.

In the present invention, a single fiber of carbon fiber and an aggregate thereof are simply referred to as carbon fiber. The aggregate of carbon fibers of the present invention includes various forms such as a bundle form, a web form, or a composite form thereof. The method for producing the carbon fiber of the present invention will be described later.

In the present invention, the tensile elastic modulus is a general term indicating a single fiber elastic modulus evaluated by a single fiber tensile test of carbon fibers and a strand elastic modulus evaluated by a method described later. The relationship between the single fiber elastic modulus and the strand elastic modulus will be described later.

A first aspect of the carbon fiber of the present invention is a carbon fiber having a strand elastic modulus of 360 GPa or more, a strand strength of 3.5 GPa or more and a single fiber diameter of 6.0 μm or more, and further the following requirements (a): ) Or (b). It is more preferable to satisfy both (a) and (b).
(A) When one end is a fixed end and the other end is a free end that is rotatable with respect to the axis of the fiber bundle, the number of remaining twists is 2 turns/m or more. (b) A single carbon fiber The total fineness, which is the product of the fiber fineness (g/km) and the number of filaments (pieces), is 740 g/km or more.
Each requirement will be described below.

In the first aspect of the carbon fiber of the present invention, the strand elastic modulus is 360 GPa or more. The strand elastic modulus is preferably 370 GPa or more, more preferably 380 GPa or more, further preferably 400 GPa or more, and further preferably 440 GPa or more. The higher the strand elastic modulus, the greater the effect of improving the rigidity of the carbon fiber when the carbon fiber reinforced composite material is formed, and it is easy to obtain the carbon fiber reinforced composite material having high rigidity. When the strand elastic modulus is 360 GPa or more, the rigidity of the carbon fiber reinforced composite material can be significantly increased, and thus the industrial value is great. From the viewpoint of increasing the rigidity of the carbon fiber reinforced composite material, the higher the strand elastic modulus of the carbon fiber, the more preferable, but conventionally, if the strand elastic modulus is too high, it leads to a decrease in the molding processability when obtaining the carbon fiber composite material. Or, when it was used as a discontinuous fiber, it was easy to lead to a decrease in fiber length. The strand elastic modulus can be evaluated according to the tensile test of the resin-impregnated strand described in JIS R7608:2004. The details of the method for evaluating the strand elastic modulus will be described later. The strand elastic modulus can be controlled by various known methods, but in the present invention, it is preferably controlled by the tension in the carbonization treatment.

In the first aspect of the carbon fiber of the present invention, the strand strength is 3.5 GPa or more. The strand strength is preferably 3.7 GPa or more, more preferably 3.9 GPa or more, and further preferably 4.3 GPa or more. Generally, the higher the strand strength, the higher the tensile strength of the carbon fiber reinforced composite material, so that a high performance carbon fiber reinforced composite material can be obtained. Carbon fibers having an extremely low strand strength may lead to a decrease in moldability when forming a carbon fiber reinforced composite material, but if the fiber strength is 3.5 GPa or more, it is not a big problem in many cases. The strand strength can be evaluated according to the tensile test of the resin-impregnated strand described in JIS R7608:2004. The details of the method for evaluating the strand strength will be described later. The strand strength can be controlled by various known methods, but in the usual method of increasing the carbonization temperature, the strand strength tends to decrease as the strand elastic modulus increases. A carbon fiber having a strand strength of 3.5 GPa or more even if the strand elastic modulus is high can be obtained by the method for producing a carbon fiber of the present invention described later.

In the first aspect of the carbon fiber of the present invention, the single fiber diameter is 6.0 μm or more. The single fiber diameter is preferably 6.5 μm or more, more preferably 6.9 μm or more. In general, the larger the single fiber diameter, the more difficult it is to achieve both high levels of strand elasticity and strand strength. However, according to the first aspect of the carbon fiber of the present invention, the single fiber diameter is Even if it is 6.0 μm or more, both can be compatible with each other at the above-mentioned high level. Further, the larger the single fiber diameter is, the more fluffing occurs due to the friction between the carbon fibers when unwinding from the bobbin and the friction with the guide member such as a roller when the carbon fiber reinforced composite material is unwound, and the accumulation of the fluff on the guide member. It is easily suppressed and the moldability is improved. In the first aspect of the carbon fiber of the present invention, the upper limit of the single fiber diameter is not particularly limited, but if it is too large, the strand strength and the strand elastic modulus are likely to decrease, so about 15 μm may be considered as the temporary upper limit. In addition, the single fiber diameter is also preferably 7.4 μm or less from the viewpoint of easily achieving a high level of both strand elastic modulus and strand strength. Although the method for evaluating the diameter of the single fiber will be described later, it may be calculated from the specific gravity of the fiber bundle, the basis weight, and the number of filaments, or may be evaluated by observation with a scanning electron microscope. If the evaluation device used is properly calibrated, the same result can be obtained by any method. When the cross-sectional shape of the single fiber is not a perfect circle when evaluated by scanning electron microscope observation, the equivalent circle diameter is used instead. The equivalent circle diameter refers to the diameter of a true circle having a cross-sectional area equal to the measured cross-sectional area of a single fiber. The single fiber diameter can be controlled by the discharge amount from the spinneret during the spinning of the carbon fiber precursor fiber bundle, the drawing ratio in each step, and the like.

A first aspect of the carbon fiber of the present invention is a carbon fiber that satisfies one or more of the following requirements in addition to the requirements regarding the strand elastic modulus, the strand strength, and the single fiber diameter described above.
(A) When one end is a fixed end and the other end is a free end that is rotatable with respect to the axis of the fiber bundle, the number of remaining twists is 2 turns/m or more. (b) A single carbon fiber The total fineness, which is the product of the fiber fineness (g/km) and the number of filaments (pieces), is 740 g/km or more. By satisfying either or both of these requirements (a) and (b), the strand elastic modulus Even if it is high, the deterioration of molding processability can be effectively suppressed and the industrial value is great.

In the first aspect of the carbon fiber of the present invention, the number of remaining twists is preferably 2 turns/m or more, more preferably 5 turns/m or more, and further preferably 10 turns/m or more. It is more preferably 16 turns/m or more, still more preferably 20 turns/m or more, further preferably 30 turns/m or more, and further preferably 46 turns/m or more.

In the present invention, the fixed end is an arbitrary portion on the fiber bundle that is fixed so as not to rotate about the longitudinal direction of the fiber bundle, and restrains the rotation of the fiber bundle using an adhesive tape or the like. Can be realized by In the present invention, the free end refers to an end portion that appears when a continuous fiber bundle is cut in a cross section perpendicular to the longitudinal direction, is not fixed to anything, and the longitudinal direction of the fiber bundle is the axis. It is the end that can be rotated. In the present invention, when one end is a fixed end and the other end is a free end, the remaining number of twists means the number of twists per 1 m of the permanent twist of the carbon fiber bundle. Semi-permanent twist refers to twist that cannot be unwound freely without the action of external force. In the present invention, one end is a fixed end and the other end is a free end, and the twist that remains undissolved after standing for 5 minutes in the specific arrangement described in the Examples is a semi-permanent twist, That is, it is defined as the remaining twist. If the number of remaining twists is 2 turns/m or more, it is easy to maintain high moldability even if the strand elastic modulus is high. The reason for this has not been clarified quantitatively, but is qualitatively understood as follows. That is, in the case of carbon fibers having a remaining twist number of 2 turns/m or more, the relative positions of the single fibers in the fiber bundle are easily fixed due to twisting, so that the single fibers inside the fiber bundle are It is considered that they are easily preserved without being damaged by friction with the guide member and the like. Further, if the number of remaining twists is 5 turns/m or more, fluff is suppressed, so that high tension can be applied in the carbonization step, and the strand elastic modulus can be effectively increased easily. When the number of remaining twists is 20 turns/m or more, the number of fluffs is small and the alignment of the fiber bundles is controlled. As a result, the stress transmission between the fiber bundles becomes smooth, and the knot strength described later tends to increase. When one end is a fixed end and the other end is a free end, the number of remaining twists can be controlled by a known method. Specifically, the number of remaining twists can be controlled by adjusting the number of twists of the fiber bundle in the carbonization treatment step.

As described above, in the first aspect of the carbon fiber of the present invention, the total fineness is preferably 740 g/km or more, more preferably 850 g/km or more, and further preferably 1300 g/km or more. It is more preferably 1600 g/km or more, further preferably 2000 g/km or more. When the total fineness is 740 g/km or more, it is easy to maintain high moldability even if the strand elastic modulus is high. The reason for this has not been clarified quantitatively, but is qualitatively understood as follows. That is, in the carbon fibers having a total fineness of 740 g/km or more, the ratio of the number of single fibers existing in the outermost layer of the fiber bundle susceptible to damage due to the friction to the total number of single fibers forming the fiber bundle becomes small. Therefore, it is considered that the above-mentioned damage due to friction is likely to be reduced in the entire fiber bundle. The total fineness is a product of the single fiber fineness (g/km) and the number of filaments (pieces), and can be controlled by changing the single fiber fineness and the number of filaments.

The second aspect of the carbon fiber of the present invention is the carbon fiber in which the single fiber elastic modulus Es (GPa) and the loop breaking load A (N) satisfy the relationship of the formula (1).
A≧−0.0017×Es+1.02... Formula (1)
The constant term in formula (1) is preferably 1.04, more preferably 1.06, even more preferably 1.08, and particularly preferably 1.10. The loop breaking load corresponds to the load at which breaking occurs when the single fiber is bent into a loop, and is evaluated by the method described below. The single fiber elastic modulus is the tensile elastic modulus of the carbon fiber as a single fiber, and has a certain correlation with the strand elastic modulus. In the present invention, the single fiber elastic modulus will be described in detail later, but a single fiber tensile test is performed with a plurality of test lengths, the slope of the stress-strain curve in each test length is calculated, and the test length dependence is considered. This can be obtained by removing the influence of the compliance of the device system. Usually, when the single fiber elastic modulus is increased, the loop breaking load tends to decrease. When the loop rupture load is low, the carbon fiber is likely to break due to the force in the bending direction during the molding process as a discontinuous fiber, and the fiber length becomes short, so that the effect of improving the rigidity of the carbon fiber reinforced composite material becomes small. The higher the loop breaking load, the less likely it is to break even when a force in the bending direction is applied to the single fiber, so the fiber length is easily maintained during molding processing as a discontinuous fiber to which a large bending force is applied. It is easy to increase the rigidity of the fiber reinforced composite material. When the loop breaking load A and the single fiber elastic modulus Es satisfy the relationship of the formula (1), the carbon fiber becomes a carbon fiber which is hard to be broken by the force in the bending direction even though the single fiber elastic modulus is high. The rigidity of the carbon fiber reinforced composite material can be efficiently increased. The carbon fiber satisfying the relationship of the formula (1) can be obtained by the method for producing carbon fiber of the present invention described later. Moreover, it is preferable that the carbon fiber which is the first aspect of the present invention simultaneously satisfies the second aspect. Even if the carbon fiber has a high strand elastic modulus, it is possible to effectively suppress the deterioration of the molding processability, and it is easy to maintain the fiber length when used as a discontinuous fiber. Easy to obtain material.

In the second aspect of the carbon fiber of the present invention, the single fiber elastic modulus is preferably 360 GPa or more, more preferably 370 GPa or more, further preferably 380 GPa or more, and further preferably 400 GPa or more. It is preferably 440 GPa or more and more preferably 440 GPa or more. Conventionally, the higher the single fiber elastic modulus, the lower the loop breaking load, and the fiber length was likely to be short during the molding process as a discontinuous fiber. However, in the second aspect of the carbon fiber of the present invention, the single fiber elasticity is Since the loop rupture load is higher than the modulus, the rigidity of the carbon fiber reinforced composite material can be effectively increased even if the single fiber elastic modulus is increased. The method for improving the single fiber elastic modulus is the same as the strand elastic modulus.

The third aspect of the carbon fiber of the present invention is that the single fiber diameter is 6.0 μm or more, and the knot strength B (evaluated at a strand elastic modulus E (GPa) and a heating weight loss rate at 450° C. of 0.15% or less. And (MPa) satisfy the relationship of the formula (2), and the twist number is 5 to 80 turns/m.
B≧6.7×10 ⁹ ×E ^−2.85 ...Equation (2)
In the third aspect of the carbon fiber of the present invention, the single fiber diameter is 6.0 μm or more. The single fiber diameter is preferably 6.5 μm or more, more preferably 6.9 μm or more. In general, the larger the single fiber diameter, the more difficult it is to achieve both high levels of both the strand elastic modulus and the knot strength. However, according to the third aspect of the carbon fiber of the present invention, the single fiber diameter is Even if it is 6.0 μm or more, both can be compatible at a high level. In addition, the larger the single fiber diameter, the more the fluffing due to the friction between the carbon fibers when unwound from the bobbin and the friction with the guide member such as the roller can be more suppressed when forming the carbon fiber reinforced composite material, and the molding process is performed. You can improve your sex. In the third aspect of the carbon fiber of the present invention, the upper limit of the single fiber diameter is not particularly limited, but if it is too large, the knot strength and the strand elastic modulus are likely to decrease, so about 15 μm may be considered as the temporary upper limit. In addition, the single fiber diameter is also preferably 7.4 μm or less from the viewpoint of easily achieving a high level of both strand elastic modulus and knot.

In the third aspect of the carbon fiber of the present invention, the strand elastic modulus E (GPa) and the knot strength B (MPa) evaluated at a heating weight loss rate at 450° C. of 0.15% or less have the relationship of the formula (2). Fulfill.
B≧6.7×10 ⁹ ×E ^−2.85 ...Equation (2)
In the present invention, the heating weight loss rate at 450° C., which will be described later in detail, is calculated from the change in mass before and after heating when the carbon fibers are heated in an oven in a nitrogen atmosphere at a temperature of 450° C. for 15 minutes. The knot strength is an index that reflects the mechanical properties of the fiber bundle other than the fiber axis direction. When manufacturing a composite material, a bending stress other than the fiber axis direction is applied to the carbon fiber bundle, and the knot strength influences the generation of fluff which is a fiber breakage occurring in the manufacturing process of the composite material. In order to efficiently manufacture the composite material, fluff occurs when the traveling speed of the fiber bundle is increased during the manufacturing of the composite material.However, by increasing the knot strength, the composite material can be produced with good quality even under the condition that the traveling speed of the fiber bundle is high. Obtainable. Such knot strength tends to improve when a sizing agent is added to the carbon fiber bundle. On the other hand, when using a matrix having a high molding temperature, when there is a concern that the adhesive strength between the carbon fiber and the matrix may be reduced due to a thermal decomposition product of the sizing agent, it may be preferable not to add the sizing agent from the viewpoint of improving the adhesive strength. is there. Therefore, in the present invention, the knot strength of the carbon fiber bundle in a state where sizing is not applied is used as an evaluation index. That is, the evaluation that the heating weight loss rate at 450° C. is 0.15% or less means that the sizing material is not applied, or the sizing material is applied and the heating weight loss rate at 450° C. exceeds 0.15%. In some cases, it is shown that the sizing material is removed before the evaluation. The sizing agent may be removed by a known method, and for example, a method of removing it with a solvent in which the sizing agent is soluble can be mentioned. When the knot strength is low, fluff is likely to be generated during the molding process into the carbon fiber reinforced composite material, and the moldability tends to decrease. Usually, the knot strength tends to decrease as the strand elastic modulus increases. When the strand elastic modulus and the knot strength satisfy the relationship of the expression (2), the strand elastic modulus and the knot strength can be compatible with each other in a high balance. The proportional constant in the formula (2) is preferably 6.9×10 ⁹ and more preferably 7.2×10 ⁹ . The carbon fiber in which the strand elastic modulus and the knot strength satisfy the relationship of the formula (2) can be obtained by the method for producing a carbon fiber of the present invention described later.

Further, it is preferable that the carbon fiber which is the first aspect of the present invention simultaneously satisfies the third aspect and/or the second aspect. Such a carbon fiber can effectively suppress deterioration of molding processability even if the strand elastic modulus is high. In particular, when yarn joining is required during molding, the yarn joining portion is less likely to break, which is advantageous for continuous production.

In the third aspect of the carbon fiber of the present invention, the twist number is 5 to 80 turns/m. When the number of twists is within the above range, the number of fluffs is small and the alignment of the fiber bundles can be controlled. As a result, stress transmission between the fiber bundles becomes smooth and the knot strength is likely to be increased. The twist number in the third embodiment is preferably 20 to 80 turns/m from the viewpoint of improving handleability during molding.

When the carbon fiber of the present invention is in the form of a carbon fiber bundle, the twist angle of the surface layer of the carbon fiber bundle is preferably 2.0 to 30.5°. The twist angle of the carbon fiber bundle surface layer is an angle formed by the fiber axis direction of the single fibers existing in the outermost layer of the carbon fiber bundle with respect to the long axis direction of the carbon fiber bundle, and is directly observed. However, it can be calculated more accurately from the number of twists, the number of filaments, and the diameter of single fiber as described later. When the twist angle is controlled within the above range, fluff is suppressed, so that high tension can be applied in the carbonization step, and the strand elastic modulus can be effectively increased easily. The twist angle of the carbon fiber bundle surface layer in the present invention is preferably 4.8 to 30.5°, more preferably 4.8 to 24.0°, and more preferably 4.8 to 12.5°. More preferably, it is more preferably 4.8 to 10.0°. A carbon fiber bundle having a twist angle satisfying the above range can be produced according to the method for producing a carbon fiber of the present invention described later. Specifically, the twist angle of the surface layer of the carbon fiber bundle can be controlled by adjusting the number of filaments and the diameter of single fiber in the carbonization step, in addition to adjusting the number of twists of the fiber bundle. The larger the number of filaments of the carbon fiber bundle and the diameter of the single fiber, the larger the twisting angle can be maintained for the fiber bundle having the same number of twists, so that the twisting effect can be further enhanced.

In the carbon fiber of the present invention, it is preferable that the crystallite size Lc (nm) and the crystal orientation degree π ₀₀₂ (%) satisfy the relationship of the formula (3).
π ₀₀₂ ≧4.0×Lc+73.2... Expression (3)
The crystallite size Lc is an index representing the thickness of the crystallite present in the carbon fiber in the c-axis direction. Usually, it is often evaluated by wide-angle X-ray diffraction of a fiber bundle, but it is evaluated for one single fiber by microbeam wide-angle X-ray diffraction, and the average of the measured values for three single fibers is taken to obtain an average crystal It may be the child size Lc(s). When the size of the microbeam is equal to or less than the diameter of the single fiber, the average crystallite size Lc(s) is a value obtained by averaging the values evaluated at a plurality of points in the diameter direction of the single fiber, and the evaluation value of the single fiber, The average value of the evaluation values obtained in the same manner for the three single fibers is adopted. The detailed evaluation method will be described later. The wide-angle X-ray diffraction data of a single fiber and the widely-known wide-angle X-ray diffraction data of a fiber bundle are equivalent, and the average crystallite size Lc(s) and the crystallite size Lc take almost the same value. As a result of examination by the inventors, the crystal orientation degree π ₀₀₂ tends to increase as the crystallite size Lc increases, and the formula (3) empirically shows the upper limit of the relationship from the data of known carbon fibers. There is. Usually, as the crystallite size Lc increases, the strand elastic modulus increases, but the strand strength, knot strength, loop breaking load, and moldability into a carbon fiber reinforced composite material tend to decrease. Further, the crystal orientation degree π ₀₀₂ strongly affects the strand elastic modulus, and the higher the crystal orientation degree, the higher the strand elastic modulus. The fact that the crystal orientation degree π ₀₀₂ satisfies the relation of the expression (3) means that the crystal orientation degree π ₀₀₂ is large relative to the crystallite size Lc, and even if the strand elastic modulus is high, the strand strength or It is possible to effectively suppress the deterioration of knot strength, loop rupture load and molding processability, and it has great industrial value. In the present invention, the constant term in the formula (3) is more preferably 73.5, further preferably 74.0. The carbon fiber satisfying the relationship of the formula (3) can be obtained by increasing the drawing tension in the carbonization step.

In the carbon fiber of the present invention, the crystallite size Lc is preferably 2.2 to 3.5 nm, more preferably 2.4 to 3.3 nm or more, and 2.6 to 3.1 nm or more. More preferably, it is particularly preferably 2.8 to 3.1 nm. If the crystallite size Lc is 2.2 nm or more, the stress load inside the carbon fiber is effectively performed, so that the single fiber elastic modulus is easily increased, and if the crystallite size Lc is 3.5 nm or less, the stress concentration causes Therefore, the strand strength, knot strength, loop breaking load, and moldability tend to be high. The crystallite size Lc can be controlled mainly by the processing time of the carbonization step and the maximum temperature.

In the carbon fiber of the present invention, the crystal orientation degree π ₀₀₂ is preferably 80.0 to 95.0%, more preferably 80.0 to 90.0%, and 82.0 to 90.0%. Is more preferable. The degree of crystal orientation π ₀₀₂ is an index representing the orientation angle with reference to the fiber axis of the crystallite present in the carbon fiber. Similar to the crystallite size, one single fiber may be evaluated by microbeam wide-angle X-ray diffraction, and the average of the measured values for three single fibers may be averaged to obtain the average degree of crystal orientation π ₀₀₂ (s). When the size of the microbeam is equal to or less than the diameter of the single fiber, the average degree of crystal orientation π ₀₀₂ (s) is obtained by averaging the values evaluated at a plurality of points in the diameter direction of the single fiber as the evaluation value of the single fiber. The average value of the evaluation values obtained in the same manner for the three single fibers is adopted. The detailed evaluation method will be described later. The wide-angle X-ray diffraction data of a single fiber and the widely-known wide-angle X-ray diffraction data of a fiber bundle are equivalent, and the average degree of crystal orientation π ₀₀₂ (s) and the degree of crystal orientation π ₀₀₂ are almost equal. To take. When the crystal orientation degree is 80.0% or more, the strand elastic modulus tends to be high. The crystal orientation degree π ₀₀₂ (s) can be controlled by the stretching tension in addition to the temperature and time in the carbonization step.

In the carbon fiber of the present invention, it is preferable that the strand elastic modulus E (GPa) and the crystallite size Lc (nm) satisfy the relationship of the formula (4).
E×Lc ^−0.5 ≧200 (GPa/nm ^0.5 )... Formula (4)
As a result of examination by the present inventors, it was found that when the carbon fiber satisfies the formula (4), the strand elastic modulus and the moldability are easily compatible at a particularly high level. The reason why the strand elastic modulus and the molding processability are easily compatible at a high level by satisfying the formula (4) has not been completely clarified, but is considered as follows. That is, as seen in the Hall-Petch equation widely used in the field of polycrystalline materials, the crystallite size Lc to the -0.5th power is an index indicating a certain strength of the material. In view of this, it can be construed that the larger Lc- ^0.5 is, the stronger the material is, and the smaller Lc- ^0.5 is, the more brittle the material is. Therefore, satisfying the expression (4) means that the product of the strand elastic modulus and the toughness of the material is a certain value or more, and the strand elastic modulus and the toughness of the material are compatible at a high level. It is thought to mean that. The carbon fiber satisfying the formula (4) can be obtained by increasing the drawing tension in the carbonization step.

In the carbon fiber of the present invention, the surface oxygen concentration O/C is preferably 0.05 to 0.50. The surface oxygen concentration is an index representing the amount of functional groups containing oxygen atoms introduced to the surface of the carbon fiber, and can be evaluated by the photoelectron spectroscopy described later. The higher the surface oxygen concentration, the better the adhesion between the carbon fiber and the matrix, and the better the mechanical properties of the carbon fiber reinforced composite material. The surface oxygen concentration O/C is more preferably 0.07 to 0.30. When the surface oxygen concentration O/C is 0.05 or more, the adhesiveness with the matrix becomes a sufficient level, and when the surface oxygen concentration is 0.50 or less, peeling of the carbon fiber surface due to excessive oxidation is suppressed, and the carbon fiber composite material. Improves the mechanical properties of. A method for setting the surface oxygen concentration O/C within the above range will be described later.

When the carbon fiber of the present invention is in the form of a carbon fiber bundle, the number of filaments is preferably 10,000 or more. The number of filaments is more preferably 15,000 or more, further preferably 20,000 or more. If the number of twists is the same, the greater the number of filaments, the greater the distance between the center axis of twist and the outer circumference of the fiber bundle, so the twist is easy to stabilize, and fluff occurs or breaks even when high tension is applied in the carbonization process. Is easily suppressed, the strand elastic modulus can be effectively increased, and the molding processability can be enhanced.

Hereinafter, the method for producing the carbon fiber of the present invention will be described.

The carbon fiber precursor fiber bundle which is the basis of the carbon fiber of the present invention can be obtained by spinning a spinning solution of a polyacrylonitrile copolymer.

As the polyacrylonitrile copolymer, not only a homopolymer obtained only from acrylonitrile but also other monomers in addition to the main component acrylonitrile may be used. Specifically, the polyacrylonitrile copolymer preferably contains 90 to 100% by mass of acrylonitrile and less than 10% by mass of a copolymerizable monomer.

Examples of monomers copolymerizable with acrylonitrile include acrylic acid, methacrylic acid, itaconic acid and their alkali metal salts, ammonium salts and lower alkyl esters, acrylamide and its derivatives, allylsulfonic acid, methallylsulfonic acid and Those salts or alkyl esters can be used.

The above-mentioned polyacrylonitrile copolymer is dissolved in a solvent in which the polyacrylonitrile copolymer is soluble, such as dimethyl sulfoxide, dimethylformamide, dimethylacetamide, nitric acid, an aqueous solution of zinc chloride, an aqueous solution of rhodanese, to prepare a spinning solution. When using solution polymerization for the production of polyacrylonitrile copolymer, if the solvent used for polymerization and the spinning solvent are the same, the resulting polyacrylonitrile copolymer is separated, the step of redissolving in the spinning solvent is It is unnecessary and preferable.

A carbon fiber precursor fiber bundle can be produced by spinning the spinning solution obtained as described above by a wet or dry-wet spinning method.

The spinning solution is introduced into a coagulation bath to coagulate, and the coagulated fiber bundle obtained is passed through a washing step, a drawing step in the bath, an oil agent applying step and a drying step to obtain a carbon fiber precursor fiber bundle. .. The coagulated fiber bundle may be stretched directly in the bath by omitting the water washing step, or may be stretched in the bath after removing the solvent by the water washing step. Stretching in the bath is usually preferably carried out in a single or a plurality of stretching baths whose temperature is controlled at 30 to 98°C. Further, a dry heat drawing step or a steam drawing step may be added to the above steps.

The single fiber fineness of the carbon fiber precursor fiber bundle is preferably 0.9 dtex or more, more preferably 1.0 dtex or more, and further preferably 1.1 dtex or more. The higher the monofilament fineness of the carbon fiber precursor fiber bundle is, the more the occurrence of fiber bundle breakage due to contact with rollers and guides is suppressed, and the process stability of the spinning process, carbon fiber flame resistance, pre-carbonization and carbonization process Easy to maintain. When the single fiber fineness of the carbon fiber precursor fiber bundle is 0.9 dtex or more, it is easy to maintain the process stability. If the monofilament fineness of the carbon fiber precursor fiber bundle is too high, it may be difficult to perform uniform treatment in the flameproofing process, the manufacturing process becomes unstable, and the dynamics of the obtained carbon fiber bundle and carbon fiber Characteristics may deteriorate. The single fiber fineness of the carbon fiber precursor fiber bundle can be controlled by a known method such as the discharge amount of the spinning solution from the spinneret and the stretching ratio.

The resulting carbon fiber precursor fiber bundles are usually in the form of continuous fibers. Further, the number of filaments per yarn is preferably 1,000 to 80,000. In the present invention, the carbon fiber precursor fiber bundle may be combined as necessary to adjust the number of filaments per one yarn of the obtained carbon fiber.

The carbon fiber of the present invention can be obtained by subjecting the above-mentioned carbon fiber precursor fiber bundle to flameproofing treatment, and then sequentially performing preliminary carbonization treatment and carbonization treatment.

The flameproofing treatment of the carbon fiber precursor fiber bundle is preferably performed in an air atmosphere at a temperature range of 200 to 300°C. The carbon fiber precursor fiber bundle is subjected to flameproofing treatment to be a flameproof fiber bundle.

In the present invention, the flame-resistant fiber bundle is pre-carbonized subsequent to the flame resistance. In the pre-carbonization step, the flame-resistant fiber bundle obtained by the flame-proofing treatment is heat-treated in an inert atmosphere at a maximum temperature of 500 to 1000° C. until the density reaches 1.5 to 1.8 g/cm ^3. Is preferred. The flame-resistant fiber bundle is pre-carbonized to be a pre-carbonized fiber bundle.

Further, following the preliminary carbonization, the preliminary carbonized fiber bundle is carbonized. In the carbonization step, the pre-carbonized fiber bundle obtained by the pre-carbonization treatment is carbonized in an inert atmosphere. The maximum temperature of the carbonization treatment is preferably 1500°C or higher, more preferably 2300°C or higher. The highest temperature in the carbonization step is preferably higher from the viewpoint of increasing the strand elastic modulus and single fiber elastic modulus of the obtained carbon fiber, and if it is 1500° C. or higher, the strand elastic modulus, single fiber elastic modulus, knot strength and loop are obtained. It is possible to obtain a carbon fiber having a high breaking load compatible with each other. On the other hand, if the carbonization temperature is too high, the knot strength and the loop rupture load tend to decrease, so the maximum temperature in the carbonization step is the required strand elastic modulus and single fiber elastic modulus, and the knot strength and the loop rupture load. It is good to decide in consideration of balance. The carbon fiber of the present invention can easily maintain the physical property balance even when the maximum temperature in the carbonization step is set to 2300°C.

In the present invention, the tension in the carbonization step is 5 mN/dtex or more, preferably 5 to 18 mN/dtex, more preferably 7 to 18 mN/dtex, and more preferably 9 to 18 mN/dtex. Is particularly preferable. The tension in the carbonization step is the total fineness (dtex) which is the product of the single fiber fineness (dtex) of the carbon fiber precursor fiber bundle used and the number of filaments, using the tension (mN) measured at the carbonization furnace outlet side. It shall be removed. By controlling the tension within the above numerical range, the crystal orientation degree π ₀₀₂ can be controlled without significantly affecting the crystallite size Lc of the obtained carbon fiber, and the above-mentioned formula (1) or/ And the carbon fiber satisfying the relationship of the formula (2) is obtained. From the viewpoint of increasing the strand elastic modulus and the single fiber elastic modulus of the carbon fiber, the tension is preferably high, but if the tension is too high, the carbonization process passability and the quality of the resulting carbon fiber may deteriorate, It is good to set it in consideration of both.

In the method for producing a carbon fiber of the present invention, it is more preferable if it is a method for producing a carbon fiber that further satisfies the following requirement (c) or (ii). It is more preferable to satisfy both (c) and (d).
(C) The number of twists of the fiber bundle to be subjected to carbonization treatment is 2 turns/m or more. (d) The total fineness, which is the product of the single fiber fineness (g/km) and the number of filaments (pieces) of the resulting carbon fiber. By satisfying these (c) or (2) at 740 g/km or more, even if the strand elastic modulus is high, the carbon fiber has excellent moldability.

In the carbon fiber of the present invention, the twist number of the fiber bundle during the carbonization treatment is 2 turns/m or more. The number of twists is preferably 5 turns/m or more, more preferably 10 turns/m or more, further preferably 16 turns/m or more, and further preferably 30 turns/m or more. , 46 turns/m or more is more preferable. The upper limit of the number of twists is not particularly limited, but about 60 turns/m or less is effective for increasing the productivity and the stretching limit in the carbonization step. By controlling the number of twists within the above range, generation of fluff is suppressed in the carbon fiber manufacturing process, and thus it becomes possible to apply high tension to the carbon fiber having high strand elastic modulus and single fiber elastic modulus. Easy to get. The number of twists of the fiber bundle being carbonized is the number of twists of the fiber bundle being carbonized. If the tension in the carbonization step is increased without imparting twist, single fiber breakage occurs, the number of fluffs increases, the passability of the carbonization step decreases, or the entire fiber bundle breaks, which is necessary. The tension may not be maintained in some cases. The number of twists is a surface orthogonal to the unwinding direction of the bobbin when the carbon fiber precursor fiber bundle, the flame-resistant fiber bundle, or the pre-carbonized fiber bundle is once wound around the bobbin, and then the fiber bundle is unwound. The method can be controlled by, for example, a method of swirling, or a method of imparting twist by contacting a rotating roller or belt with a running fiber bundle without being wound on a bobbin.

In the present invention, the number of filaments of the fiber bundle during the carbonization treatment is preferably 10,000 or more, more preferably 15,000 or more, and further preferably 20,000 or more. If the number of twists of the fiber bundle during carbonization is the same, the larger the number of filaments, the greater the distance between the central axis of twist and the outer periphery of the fiber bundle, so that the fluff suppressing effect due to the above-described twist is easily expressed, The single fiber elastic modulus of the obtained carbon fiber can be effectively increased. The upper limit of the number of filaments is not particularly limited and may be set according to the intended use.

In the present invention, as the inert gas used in the inert atmosphere, for example, nitrogen, argon and xenon are preferably exemplified, and nitrogen is preferably used from the economical point of view.

The carbon fiber bundle obtained by the above-mentioned production method may be subjected to additional graphitization treatment in an inert atmosphere up to 3000° C., and the elastic modulus of the single fiber may be appropriately adjusted according to the application.

The carbon fiber bundle obtained as described above is preferably subjected to surface treatment after carbonization treatment to introduce a functional group containing an oxygen atom in order to improve the adhesive strength between the carbon fiber and the matrix. As the surface treatment method, gas phase oxidation, liquid phase oxidation and liquid phase electrolytic oxidation are used, but liquid phase electrolytic oxidation is preferably used from the viewpoint of high productivity and uniform treatment. In the present invention, the liquid-phase electrolytic oxidation method is not particularly limited, and a known method may be used. The amount of current during electrolytic surface treatment for liquid-phase electrolytic oxidation is preferably 2 to 100 c/g, more preferably 2 to 80 c/g. If the amount of current during the electrolytic surface treatment is 2 c/g or more, sufficient oxygen-containing functional groups are introduced on the carbon fiber surface, adhesion to the resin is easily obtained, and a decrease in elastic modulus of the composite material can be suppressed, and 100 c/g When it is g or less, the formation of defects on the carbon fiber surface due to the electrolytic surface treatment can be suppressed, and the decrease in the loop rupture load can be suppressed.

By performing a surface treatment such as the electrolytic surface treatment, a functional group containing oxygen atoms can be introduced into the carbon fiber bundle, and the surface oxygen concentration O/C of the carbon fiber bundle can be adjusted. In order to control the surface oxygen concentration O/C within the preferable range of the present invention, the current amount and the treatment time in the surface treatment may be adjusted by a known method.

After such electrolytic treatment, a sizing agent may be attached in order to further improve the handleability and higher-order processability of the obtained carbon fiber bundle or to increase the adhesive strength between the carbon fiber and the matrix. The sizing agent can be appropriately selected according to the type of matrix used in the carbon fiber reinforced composite material. In addition, the amount of adhesion and the like may be finely adjusted from the viewpoint of handleability and higher workability. Furthermore, when there is a concern that the adhesive strength between the carbon fiber and the matrix due to the thermal decomposition product of the sizing agent is concerned, such as when using a matrix with a high molding temperature, the sizing adhesion amount should be reduced as much as possible, or the sizing treatment should be performed. Does not have to be performed.

The methods for measuring various physical properties described in this specification are as follows. In addition, those not particularly described were evaluated by measurement n number 1.

<Strand strength and elastic modulus of carbon fiber>
The strand strength and the strand elastic modulus of the carbon fiber are determined according to the resin impregnated strand test method of JIS R7608:2004 according to the following procedure. However, when the fiber bundle of carbon fibers has a twist, it is evaluated by untwisting by imparting the same number of twists in the reverse rotation as the number of twists. As the resin formulation, "Celoxide (registered trademark)" 2021P (manufactured by Daicel Chemical Industries, Ltd.)/3 boron monofluoride triethylamine (manufactured by Tokyo Chemical Industry Co., Ltd.)/acetone=100/3/4 (parts by mass) was used. As the curing conditions, normal pressure, temperature of 125° C., and time of 30 minutes are used. Ten strands of the carbon fiber bundle are measured, and the average value is taken as the strand strength and the strand elastic modulus. The strain range for calculating the strand elastic modulus is 0.1 to 0.6%.

<Average single fiber diameter of carbon fiber>
The single fiber cross section of the carbon fiber to be evaluated is observed with a scanning electron microscope to evaluate the cross sectional area. The diameter of a true circle having the same cross-sectional area as this cross-sectional area is calculated and used as the single fiber diameter. The N number for the calculation of the single fiber diameter is 50, and the average value is adopted. The acceleration voltage is 5 keV.

In this example, a scanning electron microscope (SEM) "S-4800" manufactured by Hitachi High-Technologies Corporation was used as the scanning electron microscope.

<Number of twists remaining when one end is a fixed end and the other is a free end>
A guide bar is installed at a height of 60 cm from the horizontal plane, and the carbon fiber bundle is fixed at any position of the carbon fiber bundle with tape to the guide bar. Cut and form the free end. Enclose the free end so that it is sandwiched between tapes, and treat it so that it does not become loose. In order to eliminate twists that return temporarily or with time other than semi-permanent twists, leave this state for 5 minutes, then rotate the free end while counting the number of times until it is completely untwisted. Record the number of turns n (turns). The number of remaining twists is calculated by the following formula. The average of three times of the above measurements is taken as the number of twists remaining in the present invention.

Number of remaining twists (turn/m)=n (turn)/0.5 (m).

<Modulus of single fiber of carbon fiber>
The single fiber elastic modulus of the carbon fiber is determined as follows with reference to JIS R7606:2000. First, a bundle of carbon fibers having a size of about 20 cm is divided into approximately four equal parts, and monofilaments are sampled in order from the four bundles, and the whole bundle is sampled as evenly as possible. The sampled monofilaments are fixed on 10, 25, 50 mm perforated mounts. An epoxy adhesive "Araldite (registered trademark)" quick-curing type manufactured by Nichiban Co., Ltd. is used for fixing, and after application, it is left standing for 24 hours at room temperature to be cured. The mount with the fixed monofilament is attached to a tensile tester, and a tensile test is performed with a strain rate of 40%/min and a sample number of 15 at gauge lengths of 10, 25, and 50 mm. In the stress (MPa)-strain (%) curve of each single fiber, the apparent single fiber elastic modulus is calculated from the slope (MPa/%) in the range of strain 0.3-0.7% by the following formula. ..

Apparent single fiber elastic modulus (GPa)=strain 0.3 to 0.7% gradient (MPa/%)/10
Then, for each of the gauge lengths of 10, 25, and 50 mm, the average value E _app (GPa) of the apparent single fiber elastic modulus is calculated, and its reciprocal 1/E _app (GPa ⁻¹ ) is the vertical axis (Y axis), The reciprocal 1/L ₀ (mm ⁻¹ ) of the gauge length L ₀ (mm) is plotted as the horizontal axis (X axis). The Y-intercept in the plot is read, and the reciprocal thereof is taken to obtain the single fiber elastic modulus after compliance correction, and this value is adopted as the single fiber elastic modulus in the present invention.

In this example, a tensile tester "Tensilon RTF-1210" manufactured by A&D Co., Ltd. was used as a tensile tester.

<Loop breaking load>
Place a monofilament about 10 cm long on a glass slide, drop 1-2 drops of glycerin in the center, and lightly twist both ends of the monofilament in the fiber circumferential direction to form a loop in the center of the monofilament. Put the cover glass on. This is installed on the stage of a microscope, and a moving image is photographed under the condition that the total magnification is 100 times and the frame rate is 15 frames/sec. While adjusting the stage each time so that the loop does not go out of sight, both ends of the looped fiber are pressed in the direction of the slide glass with fingers and pulled in the opposite direction at a constant speed, thereby straining the single fiber until it breaks. The frame immediately before breaking is specified by frame advance, and the lateral width W of the loop immediately before breaking is measured by image analysis. The single fiber diameter d is divided by W to calculate d/W. The number n of tests is 20, and the loop strength Es×d/W is obtained by multiplying the average value of d/W by the single fiber elastic modulus Es. Furthermore, multiplied by the cross-sectional area [pi] d ^2/4 obtained from a single fiber diameter, and a loop breaking load of πEs × ^d 3 / 4W.

<Heating loss rate of carbon fiber bundle at 450°C>
A carbon fiber bundle to be evaluated is cut to have a mass of 2.5 g and is wound with a case having a diameter of about 3 cm, and the mass w ₀ (g) before heat treatment is weighed. Then, the mixture is heated in an oven under a nitrogen atmosphere at a temperature of 450° C. for 15 minutes, allowed to cool to room temperature in a desiccator, and after heating, the mass w ₁ (g) is weighed. The heating weight loss rate at 450° C. is calculated by the following formula. The evaluation is performed 3 times and the average value is adopted.
Heat loss rate (%) at 450° C.=(w ₀ −w ₁ )/w ₀ ×100 (%).

<Knot strength of carbon fiber bundle>
The knot strength was measured using a carbon fiber bundle having a weight loss rate of 0.15% or less when heated at 450°C. When evaluating the sized carbon fiber bundle, the sizing agent is removed by washing in acetone and the dried carbon fiber bundle is used. After drying, the weight loss rate of the carbon fiber bundle at the time of heating at 450° C. is evaluated, and the carbon fiber bundle is repeatedly washed until it becomes 0.15% or less.

When the carbon fiber bundle has a twist, it is evaluated by untwisting by imparting the same number of twists in the reverse rotation as the number of twists. The carbon fiber bundle having a length of 150 mm is divided or combined so that the total fineness of the carbon fiber bundle is 7,000 to 8,500 dtex to obtain a carbon fiber bundle for measurement. The total fineness of the carbon fiber bundle is the product of the average fineness (dtex) of the single fibers of the carbon fiber bundle and the number of filaments. A gripping portion having a length of 25 mm is attached to both ends of the carbon fiber bundle as a test body, and a carbon fiber bundle is aligned by applying a load of 0.1×10 ⁻³ N/denier at the time of manufacturing the test body. A knot is made at one location at the midpoint of the test body, and a bundle tension test is performed at a crosshead speed of 100 mm/min during tensioning. The measurement is performed on a total of 12 fiber bundles, and the average value of 10 fibers obtained by dividing the two values of the maximum value and the minimum value is used as the measured value, and the standard deviation of the 10 fibers is used as the standard deviation of the knot strength. As the knot strength, a value obtained by dividing the maximum load value obtained by the tensile test by the average cross-sectional area value of the carbon fiber bundle is used.

<Twist angle of carbon fiber bundle surface layer>
After calculating the diameter (μm) of the entire carbon fiber bundle from the single fiber diameter (μm) and the number of filaments by the following formula, twisting of the carbon fiber bundle surface layer by the following formula using the number of twists (turns/m) Calculate the angle (°).

Diameter of entire carbon fiber bundle (μm)={(single fiber diameter) ² ×number of filaments} ^0.5
Twist angle (°) of carbon fiber bundle surface layer=atan (diameter of entire fiber bundle×10 ⁻⁶ ×π×number of twists).
<Crystallite size Lc and crystal orientation degree π _{002 of} carbon fiber bundle>
A carbon fiber bundle to be used for measurement is aligned and solidified using a collodion/alcohol solution to prepare a square column measurement sample having a length of 4 cm and a side length of 1 mm. The prepared measurement sample is measured under the following conditions using a wide-angle X-ray diffractometer.

1. Crystallite size Lc measurement/X-ray source: CuKα ray (tube voltage 40 kV, tube current 30 mA)
・Detector: Goniometer + Monochromator + Scintillation counter ・Scanning range: 2θ = 10-40°
-Scan mode: step scan, step unit 0.02°, counting time 2 seconds.

In the obtained diffraction pattern, the full width at half maximum is obtained for the peak appearing in the vicinity of 2θ=25 to 26°, and the crystallite size is calculated from this value by the following Scherrer formula.

Crystallite size (nm)=Kλ/β ₀ cos θ _B
However,
K: 1.0, λ: 0.15418 nm (wavelength of X-ray)
^{_{β 0: (β E 2 -β}} 1 2) 1/2
β _E : Apparent half width (measured value) rad, β ₁ : 1.046×10 ⁻² rad
θ _B : Bragg diffraction angle.

2. Measurement of degree of crystal orientation π ₀₀₂ Calculated using the following formula from the half-value width of the intensity distribution obtained by scanning the above-mentioned crystal peak in the circumferential direction.
π ₀₀₂ =(180-H)/180
However,
H: Apparent full width at half maximum (deg)
The above measurement is performed 3 times, and the arithmetic mean thereof is taken as the crystallite size and the crystal orientation degree of the carbon fiber bundle.

In Examples and Comparative Examples described below, XRD-6100 manufactured by Shimadzu Corporation was used as the wide-angle X-ray diffractometer.

<Average crystallite size Lc (s) and average degree of crystal orientation π ₀₀₂ (s) of carbon fiber single fiber>
A single fiber is randomly extracted from the carbon fiber bundle, and a wide-angle X-ray diffraction measurement is performed using an apparatus capable of using an X-ray μ beam. The measurement is performed using a microbeam having a wavelength of 0.1305 nm arranged in a shape of 3 μm in the fiber axis direction and 1 μm in the fiber diameter direction, while scanning single fibers in the fiber diameter direction in 1 μm steps. The irradiation time for each step is 2 seconds. The camera length, which is the distance between the detector and the sample, is set to be within the range of 40 to 200 mm. The camera length and beam center coordinates are determined by measuring cerium oxide as a standard sample. By subtracting the two-dimensional diffraction pattern measured by removing the sample from the detected two-dimensional diffraction pattern, the dark noise due to the detector and the scattering noise due to the air are canceled and the corrected two-dimensional diffraction pattern is obtained. .. By adding the corrected two-dimensional diffraction patterns at each position in the fiber diameter direction of the single fiber, the average two-dimensional diffraction pattern in the fiber diameter direction of the single fiber is obtained. In this average two-dimensional diffraction pattern, fan-shaped integration is performed at an angle of ±5° around the fiber axis orthogonal direction to obtain a diffraction intensity profile in the 2θ direction. The diffraction intensity profile in the 2 [Theta] direction least square fitting using the two Gaussian functions, and the angle 2 [Theta] _m of 2 [Theta] diffraction intensity is maximized _(°), full width at half maximum FWHM of the composite function of two Gaussian function (°) calculate. Further, circumferential integration is performed with a width of ±5° centering on the angle 2θ _m (°) when the diffraction intensity profile in the 2θ direction becomes the maximum, and the diffraction intensity profile in the circumferential direction is acquired. The full width at half maximum FWHM _β (°) is calculated by performing least squares fitting on the diffraction intensity profile in the circumferential direction using one Gaussian function. The crystallite size Lc(s) and the crystal orientation degree π ₀₀₂ (s) of the single fiber are obtained by the following formulas, and the results for each of the three single fibers are averaged to obtain the average crystallite size Lc(s) and the average crystallite. The orientation degree π ₀₀₂ (s) is calculated.

Lc(s) (nm)=Kλ/FWHMcos(2θ _m /2)
Here, the Scherrer coefficient K is 1.0, the X-ray wavelength λ is 0.1305 nm, and the full width at half maximum FWHM and 2θ _m are used by converting the unit from an angle (°) to a radian (rad).

π ₀₀₂ (s)(%)=(180−FWHM _β )/180×100(%).

In the present embodiment, the beamline BL03XU (FSBL) second hatch of SPring-8 is used as an apparatus capable of using the X-ray μ beam, and the flat panel detector “C9827DK-10” manufactured by Hamamatsu Photonics KK as a detector ( A pixel size of 50 μm×50 μm) was used.

<Surface oxygen concentration O/C of carbon fiber>
The surface oxygen concentration O/C of the carbon fiber is determined by X-ray photoelectron spectroscopy according to the following procedure. First, the carbon fiber from which the dirt adhering to the surface is removed using a solvent is cut into about 20 mm and spread on a sample support base made of copper. Next, the sample support table is set in the sample chamber and the inside of the sample chamber is maintained at 1×10 ⁻⁸ Torr. Subsequently, AlKα _1,2 is used as the X-ray source, and the photoelectron escape angle is set to 90° and measurement is performed. Note that combined the binding energy value of the main peak (peak top) of the _{C 1s} as a correction value of the peak due to the measurement time of the charged _{286.1EV, C 1s} peak area drawing a linear base line in a range of 282~296eV Seek by. Further, the O _1s peak area is obtained by drawing a straight base line in the range of 528 to 540 eV. Here, the surface oxygen concentration is calculated as an atomic number ratio from the ratio of the O _1s peak area and the C _1s peak area using a sensitivity correction value specific to the apparatus. In this example, ESCA-1600 manufactured by ULVAC-PHI Co., Ltd. was used as the X-ray photoelectron spectroscopy apparatus, and the sensitivity correction value peculiar to the apparatus was 2.33.

<Driving stability>
As a model evaluation of molding processability, running stability is evaluated as follows. A running stability evaluation unit is prepared in which five V-groove rollers having a diameter of 50 mm, a groove width of 10 mm, and a groove depth of 10 mm are fixed on a straight line at 300 mm intervals. The carbon fiber bundle to be evaluated is passed through each V groove roller of the running stability evaluation unit in a zigzag shape so as to come into contact with the upper surface, the lower surface, the upper surface, the lower surface, and the upper surface in a state where the sizing agent is not applied, and the dancer weight While applying a tension of 1 kg, the vehicle is run at a linear velocity of 10 m/min for 30 minutes. After that, the five V-groove rollers after removing the carbon fiber bundle are graded as follows according to the state of the rollers when visually inspected.
A: No carbon fiber adhered to the roller. In addition, among A, the one in which the carbon fibers were not observed to adhere to the roller even after running for 150 minutes is particularly referred to as AA.
B: Slight wrapping of carbon fibers around the rollers (1 or 2 out of 5 rollers is found).
C: Wound of carbon fiber around the roller is seen. (3 or 4 out of 5 rollers show wrapping)
D: The winding of carbon fiber around the roller is remarkable. (Wraps are seen on all five rollers)

Hereinafter, the present invention will be described in detail based on examples, but the present invention is not limited thereto.

Examples 1 to 11 and Comparative Examples 1 to 16 described below were carried out by using the conditions described in Table 1 or Table 2 in the method of implementation described in the following comprehensive examples.

[Comprehensive Example]
A monomer composition comprising acrylonitrile and itaconic acid was polymerized by a solution polymerization method using dimethyl sulfoxide as a solvent to obtain a spinning solution containing a polyacrylonitrile copolymer. The obtained spinning solution was filtered, then once discharged from the spinneret into the air and introduced into a coagulation bath consisting of an aqueous solution of dimethylsulfoxide to obtain a coagulated filament. In addition, after washing the coagulated yarn with water, it is drawn in warm water of 90° C. at a draw ratio of 3 times in a bath, and a silicone oil agent is further applied, followed by drying using a roller heated to a temperature of 160° C. Pressure steam drawing was performed at a draw ratio of 4 to obtain a carbon fiber precursor fiber bundle having a single fiber fineness of 1.1 dtex. Next, the obtained carbon fiber precursor fiber bundles are combined into four fibers to obtain 12,000 single fibers, and heat treated in an oven at 240 to 280° C. in an air atmosphere at a draw ratio of 1 to obtain a flame-resistant fiber bundle. Converted to.

[Example 1]
After obtaining the flameproof fiber bundle by the method described in the comprehensive example, the obtained flameproof fiber bundle is twisted to impart a twist of 75 turns/m and in a nitrogen atmosphere at a temperature of 300 to 800°C. In, a preliminary carbonization treatment was performed at a draw ratio of 0.97 to obtain a preliminary carbonized fiber bundle. Next, the pre-carbonized fiber bundle was subjected to carbonization treatment under the conditions shown in Table 1, and then an electrolytic solution was treated with an aqueous sulfuric acid solution at an electric quantity of 30 coulombs per 1 g of carbon fiber to obtain a surface oxygen concentration. A carbon fiber bundle having an (O/C) of 0.09 was obtained. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The moldability grade was AA, which was a very high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 2]
A carbon fiber bundle was obtained in the same manner as in Example 1 except that the number of twists was 50 turns/m and the tension during the carbonization treatment was 5.2 mN/dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The moldability grade was AA, which was a very high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 3]
A carbon fiber bundle was obtained in the same manner as in Example 2 except that the tension during the carbonization treatment was 10.2 mN/dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The moldability grade was AA, which was a very high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 4]
A carbon fiber bundle was obtained in the same manner as in Example 1 except that the number of twists was 20 turns/m and the tension during carbonization treatment was 10.3 mN/dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The moldability grade was AA, which was a very high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 5]
A carbon fiber bundle was obtained in the same manner as in Example 3 except that the number of yarns in the precursor fiber bundle was 8 and the number of single fiber was 24,000 in the comprehensive example. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The moldability grade was AA, which was a very high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 6]
A carbon fiber bundle was obtained in the same manner as in Example 2 except that the maximum temperature of the carbonization treatment was 2350°C and the tension during the carbonization treatment was 6.5 mN/dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The moldability was A, which was a high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 7]
A carbon fiber bundle was obtained in the same manner as in Example 6 except that the tension during the carbonization treatment was 9.1 mN/dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The moldability was A, which was a high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 8]
A carbon fiber bundle was obtained in the same manner as in Example 6 except that the tension during the carbonization treatment was 11.6 mN/dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The moldability was A, which was a high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 9]
A carbon fiber bundle was obtained in the same manner as in Example 5 except that the twist number was 20 turns/m and the tension during the carbonization treatment was 11.0 mN/dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The moldability grade was AA, which was a very high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 10]
A carbon fiber bundle was obtained in the same manner as in Example 9 except that the number of twists was 5 turns/m. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The moldability grade was AA, which was a very high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 11]
A carbon fiber bundle was obtained in the same manner as in Example 3 except that in the comprehensive example, the number of combined yarns in the precursor fiber bundle was set to 2 and the number of single fibers was set to 6,000. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The moldability was A, which was a high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Comparative Example 1]
A carbon fiber bundle was obtained in the same manner as in Example 1 except that the number of twists was 0 turn/m and the tension during the carbonization treatment was 5.3 mN/dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. Since the number of remaining twists was out of the range of the present invention, the grade of moldability was B, which was lower than that of Example 1. Table 2 shows the evaluation results of the obtained carbon fibers.

[Comparative example 2]
A carbon fiber bundle was obtained in the same manner as in Example 3 except that the number of twists was 0 turn/m, the tension during the carbonization treatment was 5.4 mN/dtex, and the maximum temperature was 1400°C. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. Since the number of remaining twists was out of the range of the present invention, the grade of moldability was B, which was lower than that of Example 1. Table 2 shows the evaluation results of the obtained carbon fibers.

[Comparative Example 3]
A carbon fiber bundle was obtained in the same manner as in Example 2 except that the tension during the carbonization treatment was 1.0 mN/dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. Further, although the molding processability grade was as high as A, the elastic modulus of the obtained carbon fiber was lower than that of Example 1 because the tension during carbonization treatment was out of the range of the present invention. did. Table 2 shows the evaluation results of the obtained carbon fibers.

[Comparative Example 4]
Using a carbon fiber precursor fiber bundle having a single fiber fineness of 0.8 dtex, the carbon fiber bundle was prepared in the same manner as in Example 2 except that the tension during the carbonization treatment was 10.3 mN/dtex and the maximum temperature was 1400°C. Got The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. Since the carbon fiber precursor fiber bundle having a small single fiber fineness was used, the grade of moldability was B, which was lower than that of Example 2. Table 2 shows the evaluation results of the obtained carbon fibers.

[Comparative Example 5]
A carbon fiber bundle was obtained in the same manner as in Example 2 except that the tension during the carbonization treatment was 1.0 mN/dtex and no twisting was performed. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The moldability was B, which was rather low. Table 2 shows the evaluation results of the obtained carbon fiber bundles.

[Comparative Example 6]
A carbon fiber bundle was prepared in the same manner as in Example 2 except that the carbon fiber precursor fiber bundle having a single fiber fineness of 0.8 dtex was used, the tension during carbonization treatment was 10.3 mN/dtex, and the maximum temperature was 1900°C. Got The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. Since the number of remaining twists was out of the range of the present invention, the grade of moldability was B, which was lower than that of Example 2. Table 2 shows the evaluation results of the obtained carbon fiber bundles.

[Comparative Example 7]
A carbon fiber bundle was obtained in the same manner as in Example 6 except that the tension during the carbonization treatment was set to 1.6 mN/dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The moldability was B, which was rather low. Table 2 shows the evaluation results of the obtained carbon fibers.

[Comparative Example 8]
Carbon fiber was formed in the same manner as in Example 3 except that the number of twists was 0 turns/m. In the carbonization process, the phenomenon that the yarn being ruptured repeatedly occurred, and it was difficult to collect the carbon fiber bundle.

[Comparative Example 9]
A carbon fiber bundle was obtained in the same manner as in Example 2 except that the number of twists was 0 turn/m. Although some fluff was observed in the carbonization process, a carbon fiber bundle could be collected. The obtained carbon fiber bundle had fluff and was of poor quality. Since the number of remaining twists was out of the range of the present invention, the grade of moldability was B, which was lower than that of Example 2. The evaluation results are shown in Table 2.

[Comparative Example 10]
A carbon fiber bundle was obtained in the same manner as in Comparative Example 9 except that the tension during the carbonization treatment was 3.4 mN/dtex. The passability in the carbonization step was good, and the quality of the obtained carbon fiber bundle was also good. Since the tension during the carbonization treatment was out of the range of the present invention, the elastic modulus of the obtained carbon fiber was lower than that of Example 2. In addition, since the number of remaining twists was out of the range of the present invention, the grade of moldability was B, which was lower than that of Example 2. The evaluation results are shown in Table 2.

[Comparative Example 11]
In the comprehensive example, the number of composite yarns of the precursor fiber bundle was set to 2, the number of single fibers was set to 6,000, the number of twists was set to 0 turn/m, and the tension during carbonization treatment was set to 3.4 mN/dtex. A carbon fiber bundle was obtained in the same manner as in Example 2 except that. The passability in the carbonization step was good, and the quality of the obtained carbon fiber bundle was also good. Since the tension during the carbonization treatment was out of the range of the present invention, the elastic modulus of the obtained carbon fiber was lower than that of Example 2. Since the number of remaining twists and the total fineness were out of the range of the present invention, the grade of moldability was C, which was lower than that of Example 2. The evaluation results are shown in Table 2.

[Comparative Example 12]
A carbon fiber bundle was obtained in the same manner as in Comparative Example 11 except that the number of twists was 50 turns/m. The passability in the carbonization step was good, and the quality of the obtained carbon fiber bundle was also good. Since the tension during the carbonization treatment was out of the range of the present invention, the elastic modulus of the obtained carbon fiber was lower than that of Example 2. Since the total fineness was out of the range of the present invention, the grade of moldability was B, which was lower than that of Example 2. The evaluation results are shown in Table 2.

[Comparative Example 13]
A carbon fiber bundle was obtained in the same manner as in Example 2 except that the single fiber fineness of the precursor fiber bundle was set to 0.8 dtex and the tension during the carbonization treatment was set to 3.4 mN/dtex in the comprehensive example. It was The passability in the carbonization step was good, and the quality of the obtained carbon fiber bundle was also good. Since the tension during the carbonization treatment was out of the range of the present invention, the elastic modulus of the obtained carbon fiber was lower than that of Example 2. Since the carbon fiber precursor fiber bundle having a small single fiber fineness was used, the grade of moldability was B, which was lower than that of Example 2. The evaluation results are shown in Table 2.

[Comparative Example 14]
A carbon fiber bundle was obtained in the same manner as in Comparative Example 13 except that the number of twists was 0 turn/m. The passability in the carbonization step was good, and the quality of the obtained carbon fiber bundle was also good. Since the tension during the carbonization treatment was out of the range of the present invention, the elastic modulus of the obtained carbon fiber was lower than that of Example 2. Since the carbon fiber precursor fiber bundle having a small single fiber fineness was used and the number of remaining twists was out of the range of the present invention, the grade of the moldability was D, and the stability was higher than that of Example 2. It dropped further. The evaluation results are shown in Table 2.

[Comparative Example 15]
A carbon fiber bundle was obtained in the same manner as in Comparative Example 13 except that in the comprehensive example, the number of combined yarns in the precursor fiber bundle was 2 and the number of single fibers was 6,000. The passability in the carbonization step was good, and the quality of the obtained carbon fiber bundle was also good. Since the tension during the carbonization treatment was out of the range of the present invention, the elastic modulus of the obtained carbon fiber was lower than that of Example 2. Since the carbon fiber precursor fiber bundle having a small single fiber fineness was used and the total fineness was out of the range of the present invention, the moldability grade was C, which was lower than that of Example 2. The evaluation results are shown in Table 2.

[Comparative Example 16]
A carbon fiber bundle was obtained in the same manner as in Comparative Example 15 except that the number of twists was 0 turn/m. The passability in the carbonization step was good, and the quality of the obtained carbon fiber bundle was also good. Since the tension during the carbonization treatment was out of the range of the present invention, the elastic modulus of the obtained carbon fiber was lower than that of Example 2. Since the carbon fiber precursor fiber bundle having a small single fiber fineness was used, and the number of remaining twists and the total fineness were out of the range of the present invention, the moldability was D, which was stable as compared with Example 2. Sex was further reduced. The evaluation results are shown in Table 2.

[Reference Example 1]
Table 2 shows the evaluation results of "Torayca (registered trademark)" T700S manufactured by Toray Industries, Inc. The knot strength in the sizing-applied state was 826 MPa. The moldability was B, which was rather low.

[Reference Example 2]
Table 2 shows the evaluation results of "Torayca (registered trademark)" M35J manufactured by Toray Industries, Inc.

[Reference Example 3]
Table 2 shows the evaluation results of "Torayca (registered trademark)" M40J manufactured by Toray Industries, Inc.

[Reference Example 4]
Table 2 shows the evaluation results of "Torayca (registered trademark)" M46J manufactured by Toray Industries, Inc.

[Reference Example 5]
Table 2 shows the evaluation results of "Torayca (registered trademark)" M40 manufactured by Toray Industries, Inc.

TECHNICAL FIELD The present invention relates to a carbon fiber that has both excellent tensile modulus and molding processability into a composite material, and can easily maintain the fiber length even when used as a discontinuous fiber, and a method for producing the same. Taking advantage of such characteristics, the carbon fiber bundle obtained in the present invention is suitably used for general industrial applications such as aircraft, automobile and ship members, and sports applications such as golf shafts and fishing rods.

Claims (22)

A carbon fiber having a strand elastic modulus of 360 GPa or more, a strand strength of 3.5 GPa or more, a single fiber diameter of 6.0 μm or more, and further satisfying the following requirement (a) or (b).
(A) When one end is a fixed end and the other end is a free end that is rotatable with respect to the axis of the fiber bundle, the number of remaining twists is 2 turns/m or more. (b) A single carbon fiber The total fineness, which is the product of the fiber fineness (g/km) and the number of filaments (pieces), is 740 g/km or more. The carbon fiber according to claim 1, wherein the single fiber elastic modulus Es (GPa) and the loop breaking load A (N) satisfy the relationship of the formula (1).
A≧−0.0017×Es+1.02... Formula (1) The single fiber diameter is 6.0 μm or more, the relationship between the strand elastic modulus E (GPa) and the knot strength B (MPa) evaluated at a heating weight loss rate at 450° C. of 0.15% or less satisfies the expression (2). The carbon fiber according to claim 1 or 2, wherein the twist number is 20 to 80 turns/m.
B≧6.7×10 ⁹ ×E ^−2.85 ...Equation (2) The carbon fiber according to any one of claims 1 to 3, which has a total fineness of 850 g/km or more. The carbon fiber according to any one of claims 1 to 4, which has a strand elastic modulus of 440 GPa or more. The carbon fiber according to any one of claims 1 to 5, wherein the twist angle of the surface layer of the carbon fiber bundle is 2.0 to 30.5°. The carbon fiber according to claim 6, wherein the twist angle of the surface layer of the carbon fiber bundle is 4.8 to 10.0°. The carbon fiber according to any one of claims 1 to 7, which has a single fiber diameter of 6.5 µm or more. The carbon fiber according to claim 1, having a single fiber diameter of 7.4 μm or less. The carbon fiber according to claim 1, wherein the crystallite size Lc (nm) and the crystal orientation degree π ₀₀₂ (%) satisfy the relationship of the formula (3).
π ₀₀₂ ≧4.0×Lc+73.2... Expression (3) The carbon fiber according to claim 1, which has a crystallite size Lc of 2.2 to 3.5 nm. The carbon fiber according to any one of claims 1 to 11, wherein the strand elastic modulus E (GPa) and the crystallite size Lc (nm) satisfy the relationship of formula (4).
E×Lc ^−0.5 ≧200 (GPa/nm ^0.5 )... Formula (4) The carbon fiber according to any one of claims 1 to 12, having a surface oxygen concentration O/C of 0.05 to 0.50. The carbon fiber bundle according to any one of claims 1 to 13, wherein the number of filaments is 10,000 or more. A carbon fiber in which the single fiber elastic modulus Es (GPa) and the loop breaking load A (N) satisfy the relationship of the formula (1).
A≧−0.0017×Es+1.02... Formula (1) The single fiber diameter is 6.0 μm or more, the relationship between the strand elastic modulus E (GPa) and the knot strength B (MPa) evaluated at a heating weight loss rate at 450° C. of 0.15% or less satisfies the expression (2). , A carbon fiber having a twist number of 5 to 80 turns/m.
B≧6.7×10 ⁹ ×E ^−2.85 ...Equation (2) The carbon fiber according to claim 15 or 16, which has a single fiber elastic modulus or a strand elastic modulus of 360 GPa or more. The carbon fiber precursor fiber bundle is subjected to flameproofing treatment in a temperature range of 200 to 300° C. in an air atmosphere, and the obtained flameproofed fiber bundle has a density of 1 at a maximum temperature of 500 to 1000° C. in an inert atmosphere. A method for producing carbon fibers, which comprises performing pre-carbonization by heat-treating to 0.5 to 1.8 g/cm ³ and further heat-treating the obtained pre-carbonized fiber bundle in an inert atmosphere. The single fiber fineness of the carbon fiber precursor fiber bundle is 0.9 dtex or more, the tension during the carbonization treatment is controlled to 5 mN/dtex or more, and the following (c) or (d) is satisfied, the strand elastic modulus is A method for producing a carbon fiber of 360 GPa or more.
(C) The number of twists of the fiber bundle to be subjected to carbonization treatment is 2 turns/m or more. (d) The total fineness, which is the product of the single fiber fineness (g/km) and the number of filaments (pieces) of the resulting carbon fiber. 740 g/km or more The method for producing carbon fibers according to claim 18, wherein the number of twists of the fiber bundle subjected to the carbonization treatment is 16 turns/m or more. The method for producing a carbon fiber according to claim 18 or 19, wherein the maximum temperature of the carbonization treatment is 1500°C or higher. The method for producing carbon fiber according to claim 20, wherein the maximum temperature of the carbonization treatment is 2300° C. or higher. The method for producing a carbon fiber according to claim 18, wherein the electrolytic surface treatment is performed with a current amount of 2 to 100 c/g after the carbonization treatment.